1. Field of the Invention
This is invention relates to a decanter-type centrifuge for the separation of suspended solids from a liquid medium, the centrifuge comprising a drum and a helical conveyor rotatable mounted therein, said helical conveyor having a conveyor hub and at least one helical flight.
2. Prior Art
Conventionally, a decanter-type centrifuge (hereinafter referred to as a decanter centrifuge) comprises a hollow drum of cylindrical/conical cross-section rotatably supported by bearings and having a helical conveyor therein, rotatably supported by bearings relative to the drum. Such a centrifuge is primarily used for the separation of solid particles from sludge, i.e. sludge from sewage treatment plants.
The centrifuge works by having the materials to be separated introduced within the drum through a pipe along the conveyor's axis of rotation via an inlet arrangement. As the centrifuge rotates, the introduced sludge forms a toroidal shaped volume along the inner wall of the drum. By action of the centrifugal forces, the solid particles are concentrated as a layer along the inner wall of the drum, and from there they are transported by the helical conveyor towards one end of the centrifuge. Typically, the end of the conveyor is formed as a frustro-cone with a narrow end having approximately the same diameter as the inner diameter of the toroidal-formed sludge volume, whereby the solids leave the centrifuge having a comparatively higher concentration of solids than the incoming sludge. At the end of the centrifuge, the cleaned liquid phase leaves the centrifuge through holes or special extraction means, such as a paring device.
A centrifuge has a limited peripheral speed, fixed by material properties and stresses created by the rotation, and the internal toroidal volume is limited by the maximum length of the drum, which limit is primarily governed by the tendency of increased vibrations as the operational speed gets close to a critical frequency of vibration. Critical vibration frequencies are a property, mainly fixed by the stiffness-to-weight ratio of a body. The lowest ratio for the parts of a decanter centrifuge is found at the helical conveyor.
Known decanter centrifuges often have longitudinally mounted strips along the inner wall of the drum, intended to protect the inner wall from wear by the solids in the following way. By the action of the centrifugal forces, a layer of solid particles is deposited on the wall, which layer will be out of reach of the helical conveyor and held in position against rotation relative to the drum wall by the strips. By this method, some degree of self-sealing between the helical edge and the fixed layer of solid particles will be created.
The capacity of a decanter centrifuge is mainly dependent on two properties: the maximum safe operational rotational speed, and the size of the toroidal volume of liquid and solids contained in the drum.
The functional lifetime of a decanter centrifuge is limited by wear from the solids being conveyed, partly caused by the friction created by the transport action itself, and partly caused by friction between the peripheral edge of the conveyor against the hard and often sharp particles concentrated at high density between the strips along the drum wall during the operation of the centrifuge.
As the flights of the helical conveyor are worn along their edges, the effective volume of separation is reduced accordingly, thus reducing the separation capacity of the centrifuge.
Decanter centrifuges of the foregoing type are known to have several different design features and variations.
The limitations originating from critical frequencies and vibrations have given rise to several complicated designs, e.g., letting the helical conveyor be supported by the medium to be treated instead of being supported in rigid bearings.
Danish Patent No. 15450 shows a decanter centrifuge with a helical conveyor comprising a hollow hub with flights having an overall density less than the density of the lighter phase of the medium to be treated. In this way, the influence of the stiffness/weight ratio of the conveyor on the tendency to create vibrations is eliminated, thus making it possible to increase the safe operational speed of the centrifuge.
The disadvantages of this arrangement in a centrifuge are that the bearings supporting the conveyor are flexible, thereby making it difficult to transfer the necessary torque and forces to the conveyor from the drive system, thus limiting the conveying capacity. Furthermore, the risk of having deposition of the separated material along the inner wall of the drum in a non-coaxial manner is increased, thus causing the centrifuge to be prone to vibrations.
A large number of inventions have been made to deal with wear problems, and most of these have attempted to improve the wear resistance in highly loaded wear zones.
The latest approaches have been oriented towards the flights of the conveyor. WO 93/22062 describes a decanter centrifuge with helical flights that have wear resistant rubber protection mounted at their peripheral edges in such a manner that the rubber profile seen in axial cross-section has a different angle to the axis than the flights themselves.